It is possible but CGI per se has relatively little to do with it. CGI is a stateless protocol whereby a client (web browser) request something from a server. The server delivers the requested data and there the transaction effectively ends.

So starting backwards can a server deliver a sound stream. Yes. This is internet radio type technology and exists.

Could a sever accept multiple inputs, mix them together and stream the result. Once again yes, and this is what you need to have multiple open mics on several clients.

Finally how do you get the input, and this is possibly the major issue. Unfortunately Javascript does not have a openMicStreamToServer() method, but this is effectively what you need. To access the microphone on a client you need client side code - Java or perhaps ActiveX (others?). This code would need to open the mic, make a private socket connection to your server and stream the (compressed) voice data. On the server you would be uncompressing, mixing, recompressing then streaming it out.

So you could do it. There are all sorts of issues and they are far from trivial. There are lots of people offering products that try to work in and around this sort of idea. You may find what you want at http://www.teamspeak.org/